The extraordinary raspberry-like eyes of a tiny parasitic insect have been described for the first time.

Strepsipteran insects, which live in the bodies of paper wasps, have a visual system not found in any other living creature.

The bugs see out of structures that are made up of 50 self-contained eyes with retinas behind them.

The parasite is located between segments of the abdomen of the wasp

This is very different to other insects which use compound eyes composed of hundreds of facets that each take in a separate point of visual information, creating what amounts to a mosaic of pixels.

The eyelets of strepsipteran insects, in contrast, appear to sample the world around them in a series of "chunks".

"No other insect that we know of has eyes quite like this," says Ron Hoy, professor of neurobiology and behaviour at Cornell University and co-researcher on a study that is published in the journal Science.

"The only place one may see a comparable eye structure is in the fossils of some kinds of trilobites," he says, referring to the extinct arthropods that lived in shallow seas during the Palaeozoic era.

Mating dash

The Cornell team looked specifically at the Xenos peckii parasite which lives a short and secretive life. Females are sightless and never leave their wasp host.

Each of the eyelets samples a "chunk" of the visual world

Once mature, males must leave the host to find a mate and have just hours to complete the task before they die.

Although chemical signals are released by the female to help the male find her, sight probably plays a crucial role as well.

"Sex pheromones from females probably help males to locate the general neighbourhood of a wasp with a female parasite," says Cornell researcher Dr Birgit Ehmer. "But the male presumably relies on his vision once he is close to the wasp."

She says the importance to the insect of the visual system is also apparent from the volume of optic lobes dedicated to processing visual information, which Dr Ehmer estimates to be 75% of the insect's brain.

Each of the X. peckii's eyelets is about 65 microns (65 millionths of a meter) in diameter.

Inverted image

The developing eye can clearly be seen in this pupa

"The need for a tiny insect eye to gather lots of light and bring images into sharp focus on the receptors may explain the advantage of the eyes that deliver images in chunks instead of points," says co-researcher Dr Elke Buschbeck.

"This composite lens arrangement allows the insect to have many more photoreceptors in a given area than would be possible with a compound eye.

"If you only have so much space on your head for eyes and you want to gather the most light, you want a composite lens eye."

Each eyelet would appear to produce an inverted image that the insect's neural set-up must then process and correct.

Each eyelet is separated from its neighbours by rows of fine, brush-like hairs called microtrichia. The microtrichia are believed to block stray light from reaching the eyelets in much the same way as matte-surfaced hoods around camera lenses.

"We're certainly not claiming that this insect descended from the trilobites or anything like that," comments Professor Hoy.

"What strepsipteran insects and trilobites have in common is that they are arthropods - but so are lobsters and crabs and all the other insects - and that the structures of their eyes appear analogous - at least from what one can tell from the body surface of ancient fossils."